Recent research and survey results report that it is possible that methane hydrate equivalent to a domestic gas consumption amount for 100 years is present in the area of sea around Japan. The methane hydrate is a potential resource that is abundantly present under the seabed of the deep sea and in which methane and water are solidified in a high pressure and low temperature state.
It is difficult to economically derive the methane hydrate with the present technology because productivity is low. However, on the premise that an inexpensive production method is developed and optimization of production behavior is realized, it is expected that a way of using the methane hydrate as a future energy source will be found.
As means for deriving methane gas from methane hydrate, a depressurization method and a heating method have been proposed. A combined method as a combination of the depressurization method and the heating method has also been examined as a basic concept.
A depressurization method is a technique for reducing the pressure of a methane hydrate layer, keeping methane hydrate to be in a dissociation zone, and deriving dissociated methane gas. (See, for example, Japanese Patent Application Laid-Open No. 2006-45128). This technique includes means for depressurizing a layer and means for collecting methane gas. The depressurization method does not consume fuel because an artificial heat source is unnecessary. (Natural heat transfer between layers is utilized.) Since a well for heating is also unnecessary, drilling cost can be substantially reduced. However, in the technique of the simple depressurization method, there are many characteristics that are obstacles in realizing commercialization, such as low supply speed of heat necessary for vaporization of methane hydrate, large amounts of discharged sand and water, presence of risks of re-hydration and freezing of a production fluid, and poor usage efficiency of a production facility due to a large change in production behavior.
A general heating method being researched is a method of injecting hot water into a methane hydrate layer under the seabed and dissolving methane hydrate to derive methane gas (See, for example, Japanese Patent Application Laid-Open No. 2005-21324).
In this technique, for example, a hot water producing apparatus is provided on a platform to heat the sea water and inject thermal energy into the methane hydrate layer through an injecting well. However, a large amount of development investment is necessary because a large quantity of fuel is consumed for the purpose of hot water production and the thermal energy is distributed to the methane hydrate layer through the injecting well.
A methane gas deriving method most prospective at the present point is the simple depressurization method. However, the method has the following problems that should be solved.
(A) Intensification of a depressurization degree is the one and only method of accelerating production, and intense depressurization is an absolute requirement. Therefore, there are risks of subsidence, cracking, and gas leakage of the seabed surface.
(B) Economic burden on an entire development system is large because risks of discharge of sand and water involved in the intense depressurization are high.
(C) Since the supply of heat is insufficient, there is concern of a sudden fall in productivity or a serious production failure such as re-hydration or freezing of the geology and the production flow channel.
(D) Since water is excessively sucked under an intense depressurization condition, water treatment cost increases, compaction of the methane hydrate layer is accelerated, and the permeability of a layer falls.
(E) The shape of a production behavior curve is undesirable and thus, an increase in sub-sea cost occupying most of methane hydrate development is inevitable.
(F) A recovery ratio is difficult to improve with a production behavior curve peculiar to the depressurization method.